BACKLIGHT APPARATUS WITH INCREASED REFLECTANCE

Abstract: A backlight assembly (10) includes a light source (14) for generating light and a transmissive panel (18) allowing light to pass therethrough. A reflective enclosure (20) connected to the transmissive panel (18) encloses the light source (14) therein and includes a first reflector (34) for reflecting light impinging thereon and a second reflector (30) for reflecting light impinging thereon transmitted through the first reflector (34). The first reflector (34) is positioned between the light source (14) and the second reflector (30) and has a reflectance equal to or greater than a reflectance of the second reflector (30).

Description: BACKLIGHT APPARATUS WITH INCREASED REFLECTANCE FIELD OF THE INVENTION The present invention relates to backlight systems. More particularly, the present invention pertains to a reflective enclosure for a backlight system with improved reflectance for use with a backlit display. BACKGROUND OF THE INVENTION For electronic displays to gain increased acceptance in various military and avionic applications, backlighting of such electronic displays, such as active matrix liquid crystal displays, must be improved in both light efficiency and reliability. The conventional integrating box type of backlight system that is used with liquid crystal displays has several advantages over other backlight approaches. These advantages include high lumen efficiency, high lumen output, simple construction and reasonably compact implementation. The conventional integrating box backlight approach is especially suitable for use in avionic liquid crystal display applications which require very high luminance and lumen efficiency. The main components of a conventional integrating box type of backlight system 110, Figure 2, include a light source 114 which is typically a fluorescent lamp array and a diffuser 118 used to convert the output light to a desired distribution. A reflective enclosure 120 having a reflective liner 126 is combined with the diffuser 118 to enclose the light source 114. The reflective enclosure 120 is generally fabricated of materials with high reflectance. The lumen efficiency of the integrating box backlight system is strongly related to the reflectance of the reflective enclosure. Both highly reflective diffuse and specular reflective materials can be used in a conventional integrating box backlight system. Reflector materials used within the enclosure can be characterized in terms of their absorbance and transmittance as well as their reflectance; i.e., A=l-R-T wherein A is absorbance, R is reflectance and T is transmittance. For an opaque reflector housing, where T=0, the absorbance A is equal to 1-R. There is a major difference in backlight performance between a material with reflectance of 0.90 compared to 0.99. The corresponding absorbencies are 0.10 and 0.01, which is an order of magnitude impact. The absorbance value is critical for the integrating box reflector design because of the reliance on multiple reflections of light to obtain high efficiency and good spatial uniformity at the output of the backlight system. Therefore, reflector materials for use in an integrating box type backlight system must have high diffuse or specular reflectance, or in other words, low absorbance values. An example of a diffuse reflector used for reflective enclosures in conventional backlight systems is white paint that is heavily loaded with a highly reflective powder such as barium sulfate and is applied to a substrate. An example of a specular reflector material is a silver/mylar film deposited on a substrate that has a thin transparent plastic coating. Both of these reflector materials have reflectances in the range of about 0.90 to 0.95. However, these materials are not structurally strong over a wide range of environmental conditions including temperature, temperature shock, humidity, and mechanical shock and vibration. SPECTRALON® reflectance material, which is related to teflon and manufactured by Labsphere, Inc., North Sutton, NH. is highly reflective. SPECTRALON® reflectance material has a reflectance which exceeds about 0.99 in the visible wavelength spectrum when the SPECTRALON® reflectance material has a thickness of 7mm or more. SPECTRALON® reflectance material is translucent and also extremely durable over a wide range of environmental conditions including being unaffected by moisture and high ambient temperatures. However, it is bulky, heavy and a good thermal insulator. Therefore, a reflective enclosure having a reflectance of 0.99 made of SPECTRALON® reflectance material results in a bulky, heavy backlight system that does not allow dissipation of heat generated therein. With a display element disposed in front of the conventional backlight system and adapted to be illuminated thereby, the reflective enclosure of the backlight system has the purpose of redirecting light from the light source which is not initially directed towards the display element so that the maximum amount of light available from a given light source is directed toward the display. Because the lumen efficiency of the backlight system is strongly related to the reflectance of the reflective enclosure and the above described reflective materials either provide a less than adequate durability and reflectivity or a backlight system which has weight and heat dissipation disadvantages, there is a need for a backlight system with an improved reflective enclosure to increase the reflectance of the backlight system. SUMMARY OF THE INVENTION The present invention is a backlight apparatus with improved reflectance capable of enduring a wide range of environmental conditions. The backlight apparatus includes a light source for generating light. A transmissive panel allows light to pass therethrough. A reflective enclosure connected to the transmissive panel encloses the light source therein. The reflective enclosure includes a first reflector for reflecting light impinging thereon and a second reflector for reflecting light impinging thereon transmitted through the first reflector. The first reflector is positioned between the light source and the second reflector and has a reflectance that is equal to or greater than the reflectance of the second reflector. In one embodiment of the invention, the first reflector includes a translucent reflector having a reflectance greater than 0.94. In another embodiment of the invention, the first reflector includes a sheet of SPECTRALON® reflectance material. The second reflector may include a reflector including a barium sulfate loaded material on a substrate or a reflector including a silver/mylar film on a substrate. In one particular embodiment of the invention, the sheet of SPECTRALON® reflectance material has a thickness of at least 2mm. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded cutaway perspective view of a backlight assembly in accordance with the present invention utilized with a display assembly. Figure 2 is a cross-sectional view of a prior art backlight system. Figure 3 is a cross-sectional view of the assembled backlight assembly as shown in Figure 1 in accordance with the present invention. Figure 4 is a Reflectance vs. Thickness graph for SPECTRALON® reflectance material. Figure 5 is a detailed cross sectional view of a part of the back reflector or side reflector of the backlight assembly of Figure 1. Figure 6 is a diagram showing the calculation of the resultant reflectance of the backlight assembly of Figure 1 in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention shall be described with reference to Figures 1 and 3-6. Figure 1 shows a backlight assembly 10 for use with a liquid crystal display assembly 12. Liquid crystal displays operate as light modulators and do not emit light. Therefore, liquid crystal displays rely upon ambient illumination or backlighting to provide the necessary light. Backlight assembly 10 includes a reflective enclosure 20 which includes a back reflector 22 and a side reflector 24. The backlight assembly 10 employs a tubular serpentine hot cathode fluorescent lamp 14 as a light source. Lamp 14 may be arranged in any one of a plurality of well known configurations, for example, the lamp may be "u-shaped" or straight; the present invention not being limited to those listed herein. Fluorescent lamp 14 is positioned on lamp supports 26,27 with electrode connections 16 being inserted through openings 17 of back reflector 22. The lamp supports 26,27 are attached to back reflector 22. A partially transmissive diffuser 18 is provided to scatter light from the lamp 14 so that it will illuminate the liquid crystal display assembly 12 in all directions. The partially transmissive diffuser 18, the back reflector 22, and side reflector 24 enclose lamp 14 in a reflectance cavity 25, best shown in Figure 3. The purpose of the reflective enclosure 20 is to redirect light which is not initially directed towards the display assembly 12 through diffuser 18 so that the maximum amount of light available from the given light source is directed towards the display assembly 12. Diffuser 18 may be one of several configurations known to one skilled in the art as dictated by directionality and transmissibility requirements of the display application. For example, the diffuser may be partially or highly transmissive. In order to optimize the reflectance of the integrating backlight assembly 10, the assembly 10 is designed to minimize light absorbing material thereof. As discussed in the Background of the Invention section herein, the reflective materials utilized may be characterized in term of their absorbance rather than their reflectance. For an opaque reflector housing where no transmission occurs through the reflector, the absorbance A is equal to 1-R, where R is the reflectance. Where transmission occurs through the reflector because of its translucency, the absorbance A is decreased. The absorbance value is critical for an integrating box light assembly because of the reliance of multiple reflections of the light to obtain high efficiency and good spatial uniformity at the output of the backlight. In order to minimize light absorbance, the reflectance of the reflective enclosure 120 must be increased because the reflective enclosure 120 is a major loss component in the conventional box light system 110 due to the reflector having a large surface area and the occurrence of multiple reflections and absorptions. The present invention utilizes a reflective material which is translucent and has a high reflectance such as SPECTRALON® reflectance material available from Labsphere, Inc., to minimize absorption in backlight assembly 10. SPECTRALON® reflectance material is related to teflon and is a bright white plastic material. It has a very high reflectance of about 0.99 for a 7mm thick sample. As such, the absorption is about 0.01. As shown in Figure 4, this reflectance decreases to about 0.93 at a thickness of a little more than 1mm in thickness. However, because SPECTRALON® reflectance material has a small but significant transmittance due to its translucency, its absorbance is very low. Figure 4 shows the characteristics of reflectance vs. thickness of SPECTRALON® reflectance material in the visible region. SPECTRALON® reflectance material is translucent allowing for transmission of unreflected light therethrough and has a very small chromaticity shift and is extremely durable and machinable. It is unaffected by moisture and has thermal stability to 350°C. Stability over a wide range of environmental conditions including temperature, temperature shock, humidity, and mechanical shock and vibration makes SPECTRALON® reflectance material a good candidate for use in reflective enclosures of backlight assemblies. However, SPECTRALON® reflectance material is heavy with its density equaling about 1.25 grams per ccm. In order to achieve optimum reflectance of about 0.99 from SPECTRALON® reflectance material, a 7mm thickness would be required as shown by Figure 4. A reflective enclosure of such thickness would be extremely bulky and heavy. In addition, SPECTRALON® reflectance material is a very good thermal insulator making it difficult to remove heat from the backlight. Its high coefficient of thermal expansion also increases packaging difficulty. Therefore, although a very efficient backlight assembly 10 could be achieved with a reflective enclosure 20 being fashioned from thick 7mm slabs of SPECTRALON® reflectance material, the backlight assembly would be bulky, heavy and would not allow heat generated in the backlight to be dissipated. The present invention, as further described in detail with reference to Figures 3-6 achieves a resulting reflectance of about 0.97 to 0.98 utilizing a reflective material such as SPECTRALON® while overcoming the weight and thermal disadvantages of using a 7mm thickness slab of such material. The resultant reflectance of about 0.97 to 0.98 for the backlight assembly 10 is achieved, as shown in Figure 3, by enclosing light source 14 in reflectance cavity 25 which is formed by reflective enclosure 20 and diffuser 18. Reflective enclosure 20 includes back reflector 22 and side reflector 24. Back reflector 22 and side reflector 24 are each comprised of a primary reflector 34 of a 2-3mm thick sheet of SPECTRALON® reflectance material. The primary reflector 34 of SPECTRALON® reflectance material is positioned between the light source 14 and an underlying reflector 30. The underlying reflector 30, includes an aluminum substrate 31 with a reflective material 32 such as either a barium sulfate loaded paint, silver/mylar reflective film or other high reflectance material applied thereto; the underlying reflector having a reflectance of about 0.90 to 0.94. As shown in Figure 4, a 2-3mm thick sheet of SPECTRALON® reflectance material has a reflectance of about 0.96 to 0.97. The layered combination of the primary reflector 34 of SPECTRALON® reflectance material positioned between the lamp 14 and the underlying reflector 30 achieves a resultant reflectance of about 0.97 to 0.98. Although the reflective materials listed herein achieve the listed resultant reflectance, other combinations or layers of different reflective materials or different thicknesses of materials may also be utilized in the two reflective layer approach in accordance with the present invention with improved results. However, the primary reflector must be translucent with a high reflectance usually greater than 0.94 and the underlying reflector having a reflectance either equal to or less than the reflectance of the primary reflector. For example, the sheet of SPECTRALON® may be 1.5mm in thickness with a reflectance of 0.94 and the underlying reflector may include barium sulfate paint having a reflectance of 0.94. Even though the resultant reflectance may not reach about 0.97 to 0.98, it will still have improved reflectance in combination with environmental durability. However, once the reflectance value of the material chosen for sheet 34 decreases to a value less than that of the underlying reflector, then only durability is improved. One skilled in the art will recognize that reflectance materials with appropriate characteristics other than those listed herein for the primary or underlying reflector are contemplated in accordance with the present invention and that the invention is not limited to those reflectance materials listed herein. As shown in Figure 6, impinging rays 50 from light source 14 are about 96% reflected initially at primary reflector 34 of 2mm thick SPECTRALON® material. The primary reflector 34 being translucent and of low absorption passes about 3% of the impinging rays 50 through the sheet 34 as shown by rays 52. The rays 52 are about 92% reflected from the underlying reflector 39 of barium sulfate loaded paint deposited on the aluminum substrate 31 as reflected rays 53; some of which light passes through the primary reflector 34 of SPECTRALON® reflectance material for illumination of the display disposed in front of diffuser 18. Because of the multiple reflections occurring within the reflective cavity 25 and between the primary reflector 34 and underlying reflector 30, a resultant reflectance of about 97% to 98% for the impinging rays 50 is achieved. Thus, an absorbance of 0.02 to about 0.03 characterizes the backlight assembly 10 which is a significant improvement over previous backlight assemblies in addition to improved durability. With the use of a thin SPECTRALON® sheet, the density and thermal insulating characteristics of a thick SPECTRALON® sheet are overcome. As shown in Figure 5, the primary reflector 34 of SPECTRALON® reflectance material is bonded to the underlying reflector 30, including aluminum substrate 31 and reflective material 32, by a clear bonding material 38; for example, a silicon bonding material such as a clear silicon RTV (room temperature vulcanized)bonding material. Any adequate low absorption bonding material may be utilized and the present invention is not limited to silicon bonding materials. In addition, the SPECTRALON® reflectance material may be stitched to the substrate of the underlying reflector with, for example, a silicon nylon cord or thread. Alternatively, primary reflector 34 may be attached to the underlying reflector 30 by use of mechanical fasteners leaving an air gap 36, Figure 3, between the primary reflector 34 and the underlying reflector 30. Any appropriate mechanical fasteners for achieving such coupling are contemplated in accordance with the present invention. For the reflective enclosure 20 to endure a wide temperature range, the primary reflector 34 of SPECTRALON® material is subdivided into a plurality of small area tiles 35. A plurality of small area tiles 35 are shown in Figure 5 by the dashed line optional configuration. These tiles 35 are then bonded with a compliant bonding medium 38 such as clear silicon RTV to the reflective material 32. The compliant bonding medium 38 will then accommodate the thermal coefficient of expansion mismatch between the primary reflector 34 and the underlying reflector. In addition, the thermal coefficient of expansion mismatch may be accommodated by cutting grooves on either side or both sides of the SPECTRALON® material. Such accommodation of thermal expansion is important especially when a primary reflector 34 such as SPECTRALON® reflectance material having a high coefficient of thermal expansion is utilized. The significantly higher reflectance of the reflective enclosure 20 and lower absorbance of the reflective enclosure 20 results in an increase of backlight lumen efficiency. Even higher backlight lumen efficiencies are enabled by the present invention because with reduced absorbencies by the reflective enclosure 20, reduced absorbencies of other components such as the lamp 14, lamp supporting hardware, diffusers, etc. are achieved. Because light absorption has been significantly reduced and the number of rays reflecting within the reflective cavity 25 are increased, improved uniformity and reduced imaging of the backlight lamp and mounting hardware result therefrom. Those skilled in the art will recognize that only preferred embodiments of the present invention have been disclosed herein, that other advantages may be found and realized, and that various modifications may be suggested by those versed in the art. It should be understood that the embodiments shown herein may be altered and modified without departing from the true spirit and scope of the invention as defined in the accompanying claims.

Claims: CLAIMS 1. A backlight apparatus comprising: a light source for generating light; a transmissive panel allowing light to pass therethrough; and a reflective enclosure connected to said transmissive panel for enclosing said light source therein, said reflective enclosure comprising: a first reflector, said first reflector being translucent and diffuse and having a reflectance greater than about 0.93 in the visible wavelength spectrum and an absorbance of less than about 0.01, said first reflector further being less than about 5 mm thick; and a second reflector for reflecting light impinging thereon transmitted through said first reflector, said first reflector being positioned between said light source and said second reflector and having a reflectance greater than or equal to a reflectance of said second reflector. 2. An apparatus according to Claim 1, wherein said first reflector is a bright white material which is stable over mechanical shock and vibration. 3. An apparatus according to Claim 2, wherein said first reflector has the reflectance vs. thickness characteristic as shown in figure 4. 4. An apparatus according to Claim 3, wherein said first reflector comprises a plastic material having a density approximately equal to 1.25 grams per cubic centimeter. 5. An apparatus according to Claim 4, wherein said second reflector includes a barium sulfate loaded material on a substrate. 6. An apparatus according to Claim 4, wherein said second reflector includes a silver/mylar film on a substrate. 7. An apparatus according to Claim 4, wherein said sheet first reflector has a thickness of at least 2mm. 8. An apparatus according to Claim 7, wherein said second reflector includes a barium sulfate loaded material on a substrate. 9. An apparatus according to Claim 7, wherein said second reflector includes a silver/mylar film on a substrate. 10. An apparatus according to Claim 1 , wherein said first reflector includes a plurality of tiles bonded to said second reflector with a compliant bonding material. 11. An apparatus according to Claim 1 , wherein said first reflector includes a plurality of tiles of SPECTRALON® reflectance material bonded to said second reflector with a compliant bonding material. 12. An apparatus according to Claim 1, wherein said apparatus further includes fastener means for attaching said first reflector to said second reflector such that an airspace exists therebetween. 13. An apparatus according to Claim 1, wherein said apparatus further includes a translucent and low absorption bonding material between said first reflector and said second reflector. 14. An apparatus according to Claim 2, wherein said first reflector includes a material of greater durability than said second reflector over a range of environmental conditions, including temperature, temperature shock, humidity, and mechanical shock and vibration. 15. An apparatus according to Claim 1, wherein said first reflector comprises SPECTRALON® reflectance material. 16. A reflective surface comprising: a first reflector having a reflectance greater than 0.93 and an absorbance of less than about 0.01, said first reflector being translucent, diffuse and less than 5 mm thick; and a second reflector positioned in close proximity behind said first reflector such that said second reflector reflects light transmitted through said first reflector. 17. The reflective surface according to claim 16 wherein said first reflector has the reflectance vs. thickness characteristic as shown in figure 4. 18. The reflective surface according to claim 17 wherein said first reflector has a thickness between 1.5 mm and 5.0 mm. 19. The reflective surface according to claim 18 wherein said second reflector comprises a barium sulfate loaded material on a substrate. 20. The reflective surface according to claim 18 wherein said second reflector comprises a silver/mylar film on a substrate.

Inventors: BROWN PETER A, SYROID DANIEL D, BROWN, PETER, A., SYROID, DANIEL, D.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1994
Main classification: F21V7
IPC codes: F21V7